Iron Phosphine Catalyzed Cross-Coupling of Tetraorganoborates and Related Group 13 Nucleophiles with Alkyl Halides

Abstract

Iron phosphine complexes prove to be good precatalysts for the cross-coupling of alkyl, benzyl, and allyl halides with not only aryl triorganoborate salts but also related aluminum-, gallium-, indium-, and thallium-based nucleophiles. Mechanistic studies revealed that while Fe(I) can be accessed on catalytically relevant time scales, lower average oxidation states are not formed fast enough to be relevant to catalysis. EPR spectroscopic studies reveal the presence of bis(diphosphine)iron(I) complexes in representative catalytic reactions and related processes with a range of group 13 nucleophiles. Isolated examples were studied by Mossbauer spectroscopy and single-crystal X-ray structural analysis, while the electronic structure was probed by dispersion-corrected B3LYP DFT calculations. An EPR study on an iron system with a bulky diphosphine ligand revealed the presence of an S = 1 / 2 species consistent with the formation of a mono(diphosphine)iron(I) species with inequivalent phosphine donor environments. DFT analysis of model complexes allowed us to rule out a T-shaped Fe(I) structure, as this is predicted to be high spin. ■ RESULTS AND DISCUSSION Iron Phosphine Complexes for the Coupling of BR 4 − Nucleophiles. We previously found that the preformed Special Issue: Catalytic and Organometallic Chemistry of Earth-Abundant Metals

The Persistent Radical Effect in Organic Synthesis

“…[151] It was later experimentally shown that an iron(I) complex is ac ompetent catalyst for such cross-couplings. [152] Based on computational studies,t he groups of Gutierrez [153] and Nakamura and Morokuma [154] independently proposed two slightly different mechanisms for such iron cross-couplings.It was suggested in both cases that an iron(I) species first reductively cleaves the alkyl halide (Figure 31 b, step A) to give an iron(II) intermediate and aC radical. TheF e II complex is able to trap the transient alkyl radical (step C) to generate an iron(III) complex, which then undergoes reductive elimination (step D) to liberate the coupling product, thereby regenerating the starting iron(I) species.…”

Iron Catalysis: Historic Overview and Current Trends

“…Carbon-carbon bonds through iron catalysis are mainly formed either through coupling of a nucleophile with an electrophile (Scheme 2) or through cross dehydrogenative coupling (CDC, Scheme 3). The general pattern of coupling a nucleophile with an electrophile is shown in Scheme 2 together with some notable iron-catalyzed examples [40][41][42]. This chemistry will be described further by Robin Bedford in [29].…”

Subtle effects of ligand backbone on the efficiency of iron-diphos catalysed Negishi cross-coupling reactions

“…2 Bedford et al 3 and Nakamura et al 4 have reported Fe-diphos catalysed Negishi couplings and recently Fe-monophos catalysed Negishi couplings. 6 The mechanisms of Fe-catalysed cross-coupling reactions have been the subject of much discussion and an explanation of the ligand effects has yet to emerge. 6 The mechanisms of Fe-catalysed cross-coupling reactions have been the subject of much discussion and an explanation of the ligand effects has yet to emerge.…”